Canon Law (CIC) generally does not allow prenuptial agreements. “A marriage subject to a condition about the future cannot be contracted validly” (CIC 1102 §1). The Canon Law Letter & Spirit explains that a condition may be defined as ”a stipulation by which an agreement is made contingent upon the verification or fulfillment of some circumstance or event which is not yet certain” and goes on to state that “any condition concerning the future attached to matrimonial consent renders marriage invalid.”
You may want to discuss with your pastor or a canon law attorney other ways to provide for the interests of your children that would not invalidate your marriage.